This application claims the priority of Japanese Patent Application No. 2001-19943 filed on Jan. 29, 2001, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a double-wavelength aperture restricting filter which, for two systems whose numerical apertures and wavelengths of light in use differ from each other, can pass therethrough the wavelengths of light in use as luminous fluxes corresponding to their respective desirable numerical apertures; and an optical pickup apparatus using the same, which can efficiently converge the respective wavelengths of light in use onto their corresponding optical recording media when recording or reproducing information.
2. Description of the Prior Art
In recent years, various kinds of optical recording media have been under development, and optical pickup apparatus which can carry out recording and reproducing while using a plurality of kinds of optical recording media in common have been known. For example, a system which carries out recording and reproducing of DVD (digital versatile disc) and CD-R (recordable optical disc) by using a single optical pickup apparatus has been known.
In such two kinds of optical recording media, for example, visible light at about 650 nm is used for DVD in order to improve the recording density, whereas near-infrared light at about 780 nm is required to be used for CD-R since it has no sensitivity for light in the visible region. An optical pickup apparatus which can be used in common for both of them is based on so-called dual-wavelength beam type which uses two light beams having wavelengths different from each other as irradiation light. Further, it is necessary for the above-exemplified two optical recording media to have respective numerical apertures different from each other due to differences in characteristics therebetween. For example, DVD and CD-R have respective numerical apertures of 0.6 and 0.45.
FIGS. 8A and 8B are views schematically showing the foregoing configuration. FIG. 8A shows a case where a DVD 59a is disposed at a predetermined position (on a turntable) so as to carry out its recording/reproducing. It is a system in which a laser beam having a wavelength of 650 nm (xcex1) in a substantially parallel light state passes through an aperture stop 52 and is converged onto a recording surface of the DVD 59a by an objective lens 58, yielding a numerical aperture of 0.6. FIG. 8B shows a case where a CD-R 59b is disposed at a predetermined position (on a turntable) so as to carry out its recording/reproducing. It is a system in which a laser beam having a wavelength of 780 nm (xcex2) in a relatively divergent state passes through the aperture stop 52 and is converged onto a recording surface of the CD-R 59b by an objective lens 58, yielding a numerical aperture of 0.45.
Thus, optical pickup apparatus of double-wavelength beam type require means for switching numerical apertures for two systems whose numerical apertures and wavelengths of light in use differ from each other depending on the difference in kinds of optical recording media 59. Known as an example of such means is one using a filter provided with a dichroic coat or using a liquid crystal filter so that its wavelength selectivity changes between the center part and outer peripheral part. Also, for example, there has been a technique in which the aperture stop 52 is formed like a sheet provided with two openings corresponding to respective optical recording media and their corresponding wavelengths of light in use, such that the respective openings are mechanically selected for the individual optical recording media and their corresponding wavelengths of light in use.
When the numerical aperture of the system is adjusted by the above-mentioned liquid crystal shutter or switching technique, the mechanism is likely to become complicated, thus opposing the demands for making the system compact and inexpensive. Known as the above-mentioned dichroic coat is one whose outer peripheral part is processed so as to transmit light having a wavelength of 650 nm therethrough but reflect light having a wavelength of 780 nm. In such a filter, however, there is a fear of the reflected light becoming noise.
In view of the foregoing circumstances, it is an object of the present invention to provide a double-wavelength aperture restricting filter which, for two systems having respective numerical apertures and wavelengths of light in use different from each other, can pass therethrough the respective wavelengths of light in use as luminous fluxes corresponding to their corresponding desirable numerical apertures by a simple configuration.
It is another object of the present invention to provide an optical pickup apparatus using the same.
The present invention provides a double-wavelength aperture restricting filter comprising a plane-parallel plate having at least one surface provided with a zonal part adapted to act on two wavelengths of light different from each other such that one wavelength xcex1 of light keeps the quantity of light in an outer peripheral part thereof whereas the other wavelength xcex2 of light apparently extinguishes the quantity of light in the outer peripheral part by an interference effect;
the zonal part having such a step that light supposed to pass a position corresponding to the zonal part if the zonal part does not exist and light passing a position at which the zonal part is formed if the zonal part exists generate therebetween a phase difference satisfying the following conditional expressions (1) and (2):
xcex94xcex1=mxcex1+xcex41xe2x80x83xe2x80x83(1)
xcex94xcex2=(2n+1)xcex2/2+xcex42xe2x80x83xe2x80x83(2)
where
xcex94xcex1 is the phase difference at the wavelength xcex1 between the light supposed to pass the position corresponding to the zonal part if the zonal part does not exist and the light passing the position at which the zonal part is formed if the zonal part exists;
xcex94xcex2 is the phase difference at the wavelength xcex2 between the light supposed to pass the position corresponding to the zonal part if the zonal part does not exist and the light passing the position at which the zonal part is formed if the zonal part exists;
m and n are integers;
xcex41 is the amount of shift from an integer multiple of the wavelength xcex1, where |xcex41|xe2x89xa60.25xcex1; and
xcex42 is the amount of shift from an integer multiple plus a half wavelength of the wavelength xcex2, where |xcex42|xe2x89xa60.25xcex2; and
wherein, when the wavelengths xcex1 and xcex2 of light are outputted as luminous fluxes having first and second diameters, respectively, a step boundary of the zonal part forms a circle having a diameter residing between the first and second diameters.
Preferably, the double-wavelength aperture restricting filter further satisfies the following conditional expressions (3) and (4):
|xcex41|xe2x89xa60.2xcex1xe2x80x83xe2x80x83(3)
|xcex42|xe2x89xa60.2xcex2xe2x80x83xe2x80x83(4)
The double-wavelength aperture restricting filter may be formed from a plastic material.
In the double-wavelength aperture restricting filter, the wavelength xcex1 may be 650 nm used for recording/reproducing DVD, whereas the wavelength xcex2 may be 780 nm used for recording/reproducing CD-R.
The present invention provides an optical pickup apparatus comprising, successively from a light source side, the above-mentioned double-wavelength aperture restricting filter and an objective lens for optical recording media, in which, for a first optical recording medium corresponding to a first numerical aperture and the wavelength xcex1 of light, and a second optical recording medium corresponding to a second numerical aperture and the wavelength xcex2 of light, the respective wavelengths of light are converged at desirable positions when recording or reproducing information.